Method and apparatus for controlling resistance welding

ABSTRACT

Method of controlling resistance welding to obtain high quality welds in which the pressure between the electrodes (2a, 2b) holding the materials (1a, 1b) to be welded is controlled so that the resistance value between the two electrodes (2a, 2b) is a predetermined value, and the welding current is regulated so that the voltage between the electrodes is a predetermined value, and the time during which the welding current flows is controlled to a fixed period.

FIELD OF THE INVENTION

The present invention relates to a method for controlling resistancewelding and an apparatus for controlling the resistance weldingaccording to the method. Particularly, the invention relates to a methodand apparatus for controlling resistance welding to make the quality ofwelding consistent and to improve the quality by adjusting weldingconditions such as pressure, welding current, welding time and so forhtdepending upon the condition of the material to be welded, especially atthe welding site.

BACKGROUND OF THE INVENTION

In conventional resistance welding, for example spot welding, generally,the welding strength between the welded members is largely dependentupon the electrode pressure, the size and shape of the electrode tips,the pressure-related wear on the electrodes, and the quality or wear ofthe welded members. Therefore, it is impossible to obtain consistentwelding quality even if the electrode pressure, welding current, andwelding time are maintained at constant levels. The term "electrodepressure" used herein refers to the pressure between the electrodeswhile pinching the welded members.

To cope with this problem, various monitoring systems have previouslybeen devised, such as electrode voltage systems, electrode tipresistance systems, and ultrasonic wave systems. However, since thesemonitoring systems can only roughly determine the quality of the weldedportions after completion of the welding and their respective fields ofapplication differ, they cannot positively ensure the quality of thewelded portions. (The term "quality of the weld" refers to the size andpenetration rate of the nugget formed at the weld site, the tensile andshearing strengths of the welds thereby provided, and the like.)Accordingly, even when the various prior art resistance welding systemsor various monitoring systems are used in combination, there can arisecases in which the quality of weld is poor, making it necessary torepair the weld or discard the product, depending upon the case.

Thus, a controlling system has recently been proposed to overcome thisdifficulty which automatically ensures the quality of the resistanceweld in the process of welding by controlling the electrode pressure orboth the electrode pressure and the welding current so that the voltagebetween the electrodes (including the electrode tips in the case of spotwelding) pinching the welded members can be adjusted in accordance witha predetermined reference voltage curve, the instantaneous voltage beingchosen to provide a high-quality weld, on the basis that the abovevoltage has a close relation to the quality of the weld. However, withthis system alone, it is impossible to control the size of the nugget ofthe weld optimally at all times. For instance, in th case where theelectrodes are worn by pressure, the diameter of the nugget will becomeexcessively large resulting in an excessively strong weld and greaterpower consumption than necessary. On the other hand, depending upon thewelding conditions, such as the condition of contact surfaces of thewelded members, it may happen that, even if the electrode voltage variesin accordance with the reference voltage curve, the required diameter ofnugget cannot be obtained due to insufficient current path area.

The present invention is intended to solve the above problem and toautomatically ensure the desired quality of the weld without excess anddeficiency at all times during the welding process.

SUMMARY OF THE INVENTION

For this purpose, the following fact has been confirmed throughexperiments and is applied to the controlling of resistance welding: theresistance between the electrodes clamping the welded members whileconducting the welding current has a close relationship to the contactsurfaces of the welded members, i.e. the current path area, and thecurrent path area of the weld during the welding process can be derivedfrom the resistance between the electrodes. And, it has been confirmedthat the voltage across the electrodes has a close relationship with thetemperature of and heat generation at the weld site and the voltagecurve representing the time variation of the electrode voltage reflectsthe kinds, shapes and thicknesses of the welded members and thus it ispossible to select an optimum voltage curve providing good penetrationat the weld site from these factors. Furthermore, it has been alreadyconfirmed that the most effective electrode voltage, among theseelectrode voltage curves for welding is a voltage above a predeterminedlevel, and the integral and time variation of the voltage above thepredetermined level controls the quality of the weld.

The present invention is based on the above experimental results. Beforeapplication of the welding current to initiate welding, a relativelysmall current is applied to the electrodes in order to be able tocontrol the welding pressure between the electrodes in such a mannerthat the resistance between the electrodes pinching the welded membersis adjusted to coincide with a predetermined reference resistance. Thewelding pressure is corrected and controlled during the subsequentapplication of the welding current during the welding process so thatthe resistance between the electrodes clamping the welded members willcoincide with the instantaneous resistance of a reference resistancecurve, and at the same time, the welding current is controlled so thatthe electrode voltage varies in accordance with a predeterminedreference voltage curve, and the welding time is controlled by cuttingoff the welding current at a time when the integral value of theelectrode voltage has reached a predetermined reference integral value.Or alternatively, the welding time can be controlled by cutting off thewelding current when the above electrode voltage exceeds a predeterminedlevel voltage and when the integral value of the differential voltagereach a predetermined reference integral value. As a result of thiscontrol of the welding pressure between the electrodes and of thewelding time, even as the condition of the contact surfaces of thewelded member, the pressure-related wear on teh electrode tips and thelike change the desired weld quality can also be assured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of a typical example of the electrode voltage-timecurve in the case where a mild steel plate is spot-welded.

FIG. 2 is a graph of a typical example of the electrode resistance-timecurve.

FIG. 3 is a graph of the relationships between the current path diameterand the nugget diameter and welding time.

FIG. 4 is a graph of the relationship between the reciprocal of currentpath area and the current path diameter and electrode clamping force inthe initial stage of welding current supply.

FIG. 5 is a graph illustrating the variation with time of therelationship between the electrode resistance and the reciprocal of thecurrent path area during welding under various welding conditions.

FIG. 6 is a graph illustrating the relationship between the electroderesistance and the reciprocal of the current path area at the initialcurrent supply stage under various welding conditions.

FIG. 7 is a block diagram illustrating the configuration of a firstembodiment of the present invention.

FIG. 8 is a block diagram illustrating a concrete example of the weldingpressure control circuit 14 of FIG. 7.

FIG. 9 is a block diagram illustrating the configuration of a secondembodiment of the present invention.

FIG. 10 is a block diagram illustrating the configuration of a thirdembodiment of the present invention.

FIG. 11 is a fragmentary diagram of a phase conversion data table storedin the data bank of the phase conversion data outputting circuit of 10.

FIG. 12 is a block diagram illustrating the selection of the tablevalues of the phase conversion data table in FIG. 11 for the purpose offurther explanation thereof.

FIG. 13 is a block diagram illustrating the configuration of anembodiment of the electrode exchange-time display system according tothe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Now, details of the present invention will be described with referenceto the accompanying drawings.

FIG. 1 illustrates a typical example of an electrode (tip)voltage-welding time curve (hereinafter referred to simply as "a voltagecurve") in the case where a mild steel plate of 0.8 mm thick isspot-welded. And, if the mean diameter of the contact surface betweenthe free end of the electrode tips and the welded members (hereinafterreferred to as "electrode tip diameter") is denoted by De, the electrodepressure by P, and the welding current by I, then the curve a shows thevoltage curve in the case where De=6.8 mm, P=380 kg, and I=12000 A,while the curve b shows that of the case where De=4.8 mm, P-=190 kg, andI=7800 A. The welding time is given interms of the number of cycles ofthe alternating current employed. As is apparent from the figure, thereis not a large difference between the electrode voltages shown in thecurves a, b even though the electrode tip diameters are quite different.

FIG. 2 illustrates a typical example of an electrode resistance-weldingtime curve (hereinafter simply referred to as "resistance curve" in thecase where welding is carried out under the same conditions as inFIG. 1. As is apparent from this figure, the electrode resistance isgreatly influenced by the electrode tip diameter, and as shown by thecurve a in comparison to the curve b, the electrode resistance tends todecrease as the electrode tip diameter increases.

The curves of FIG. 3 each represent the relation between either thediameter of the current path between the welded members or the diameterof nugget, and the welding time, wherein the curve a-1 shows a currentpath diameter-welding time curve in the case where De=6.8 mm, P=380 kgand I=12000 A, the curve a-2 shows a nugget diameter-welding time curveunder the same conditions, while the curve c-1 shows a current pathdiameter-welding time curve in the case where De=4.8 mm, P=190 kg andI=6000 A and the curve c-2 shows a nugget diameter-welding time curveunder the same conditions. From this figure, it is apparent that incases in which a nugget is formed, there is a close relationship betweenthe nugget diameter and the current path diameter and the relationshipis greatly influenced by the electrode tip diameter.

FIG. 4 shows the relationships between the reciprocal l/S of currentpath area S and between the current path diameter and the electrodepressure at the initial stage of the welding current supply (during thefirst cycle after the onset of current supply). From this figure, itwill be found that there exists a high correlation between the electrodepressure and the current path area or the current path diameter within adispersion range and the current path area can be controlled by theelectrode pressure at the initial current supply stage.

FIG. 5 shows the time variation of the relationship between theelectrode resistance and the reciprocal l/S of the current path areaduring welding under various welding conditions where the shape and sizeof electrode tips, electrode pressure, welding current etc. differ. Aswill be noted from the figure, the electrode resistance varies with timeduring the welding process. The direction of the arrows in the curvesindicates that direction in which time elapses. The electrode resistanceand the reciprocal of current path area are substantially proportionalto each other after the electrode resistance passes a maximum values andunder any conditions the relationship therebetween approaches the samestraight proportional line as shown by the dashed line.

FIG. 6 shows the relationship between the electrode resistance and thereciprocal l/S of the current path area at the initial current supplystage and under various welding conditions. It will be noted that inthis case also there exists a proportional relationship along asubstantially straight proportional line shown by the dashed line. Fromthis, it can be easily understood that the electrode resistance R isgiven by R=w.l/S, wherein ω is the specific resistance of weldedmembers, l is the distance between the electrodes, S is the current patharea between the welded members.

The facts indicated by the graphs show that the current path areabetween the welded members can be easily estimated during the weldingprocess by measuring the electrode resistance. Meanwhile, in this case,even if there is another weld site near the weld in question, it isstill possible to estimate the current path area by means of theelectrode resistance with almost the same certainty. Furthermore, theelectrode voltage and resistance include the voltage and resistancebetween the electrode tip and the members being welded in addition tothose between the members themselves. However, since the former aregenerally as small as 20-30 percent of the latter and are approximatelyconstant with respect to time, it is possible to regard the electrodevoltage and resistance as the values extant between the welded members.

The above facts are always true regardless of the shape and size of thefree ends of the electrodes or the kinds of welded members, and there isno fundamental change in this tendency even if the thickness or numberof the welded members differs. Accordingly, it is possible to detect thecurrent path area of the welded members during the welding process bydetecting the electrode resistance during the welding process. And,since the current path area has a close relationship to the size of thenugget formed, it is possible to obtain a desired weld quality (thedesired nugget diameter) by predetermining a reference resistance curvein accordance to which will be formed a current path area which resultsin the desired nugget diameter and by controlling continuously or at aspecified points in time the electrode pressure such that the electroderesistance coincides with the instantaneous resistance value accordingto the reference resistance curve during the welding process.

Furthermore, since it is already apparent that it is possible to detectthe degree of heating between the welded members, i.e., the rate ofnugget formation (rate of penetration) depending upon the electrodevoltage, it is possible to obtain a more consistent weld quality bycontrolling the welding current such that the integral value of theelectrode voltage follows a voltage curve which corresponds to theappropriate nugget formation rate, in addition to the above control ofelectrode pressure during the welding process. Control of the weldingcurrent supply time can be performed by means for cutting off thewelding current after a predetermined proper supply time, i.e. apredetermined number of current cycles; however this can be morereliably controlled if the welding current is cut off when the integralvalue of the differential voltage, integrated over the time when theelectrode voltage exceeds a predetermined reference voltage, reaches apreset desired value.

Now, exemplary embodiments of control system according to the presentinvention will be described with reference to the accompanying drawings.

FIG. 7 is a block diagram illustrating the configuration of a firstembodiment of the present invention. In the figure, the referencenumerals 1a, 1b denote welded members such as mild steel plates, thereference numeral 2a a movable electrode connected to a piston 3, andthe reference numeral 2b a fixed electrode. During the welding process,the welded members 1a, 1b are clamped and compressed by the electrodes2a, 2b, and welding current is fed through a transformer 4 from analternating current source 15 to the welded members. The referencenumeral 5 denotes a low-current power source for supply a small currentof intensity sufficient to detect the contact resistance between theelectrodes 2a, 2b without fusing the welded members 1a, 1b to theelectrodes 2a, 2b before the supply of the welding current, and thelow-current power source applies a specified small current across theelectrodes 2a, 2b before the supply of the welding current from thetransformer 4 to the welded members 1a, 1b. Concrete examples of thepower source 5 include a small-sized alternating welding power source ora high frequency power source of proper frequency.

The reference numeral 6 denotes a voltage sensor circuit whichcontinuously detects and rectifies the voltage between the electrodes2a, 2b (hereinafter referred to as "electrode voltage") during supply ofthe small current or the welding current. The reference numeral 7denotes a waveform reference point-holding circuit which holds the peakvoltage of each half wave of the detected voltage for the period ofone-half cycle or for a predetermined period, the reference numeral 8a,a current sensor circuit which continuously detects and rectifies thewelding current, the reference numeral 8b, a current sensor whichcontinuously detects and rectifies the small current, and the referencenumeral 9, a waveform reference point holding circuit which holds thepeak value of each half wave of the detected current of the currentsensor circuits 8a, 8b for a period of one-half cycle or for apredetermined period. The reference numeral 10 denotes a resistancecalculating circuit which calculates the resistance value across theelectrodes 2a, 2b (hereinafter referred to as "electrode resistance") bydividing the voltage value held in the waveform reference point holdingcircuit 7 by the current value held in the waveform reference pointholding circuit 9. The reference numeral 11 denotes a referenceresistance value generating circuit, which holds a predeterminedreference resistance curve, which reflects the time variation of theelectrode resistance in the case where the desired weld quality isachieved, before or during welding current supply and outputscontinuously or at predetermined times the instantaneous resistancevalue indicated by the curve after the onset of welding current supplyin synchronism with timing signal Tp from a current supply time controlcircuit 17 which will be described later. Meanwhile, the term "referenceresistance curve" herein used is not necessarily limited to smooth,continuous curves, but may include curves obtained by connecting manyresistance values at specific points in time, and the specific points intime may also be a single point, for example, that of the initial stageof the current supply.

The reference numeral 12 denotes a differential amplifier, whichcompares the voltage corresponding to the electrode resistance derivedfrom the calculation in the resistance value calculating circuit 10 tothe voltage corresponding to the instantaneous resistance value from thereference resistance curve from the reference resistance valuegenerating circuit 11 and outputs a signal corresponding to thedifferential voltage. The reference numeral 13 denotes an amplifier foramplifying the output signal from the differential amplifier 12, thereference numeral 14, a welding pressure control circuit which receivesthe output signal from the amplifier 13 and controls the piston 3 so asto control the welding pressure between the electrodes 2a, 2b. Thereference numeral 16 denotes a current control circuit which isconnected to the output side of the alternating current power source 15and outputs a welding current in accordance with the condition of theweld site. The reference numeral 17 denotes a current supply timecontrol circuit, which functions in cooperation with a welding currentsupply time automatic control system described later, but may bearranged such that current supply start and stop signals are sent to thecurrent control circuit 16 so as to output the welding current only fora predetermined necessary period of time. Meanwhile, the current supplytime control circuit 17 outputs a timing signal Tp for synchronizing theother circuits after the onset of the current supply via leads shown bydashed lines.

According to the above functions, before and during welding currentsupply, the electrode resistance value and the resistance value derivedfrom the reference resistance curve are compared by the differentialamplifier 12 every half-cycle or every predetermined specific time, andthe welding pressure control circuit 14 is activated in response to asignal corresponding to the difference to control the welding pressurebetween the electrodes 2a, 2b by means of the piston 3 so that thedifference between the above resistance values will be zero. If thewelding pressure increases, then the electrode resistance decreases,while if the welding pressure decreases, then the electrode resistanceincreases. Therefore, with the electrode resistance controlled duringthe supply of current so as to conform to the above reference resistancecurve within an allowable range or coincide with the resistance valuederived from the reference resistance curve within an allowable range atleast at predetermined specific points in time, the current path areabetween the welded members 1a, 1b can be ensured.

Concerning the order of activation, first a small current is supplied bythe small current generating power source 5 before the welding current,or before carrying out the actual welding, to detect the contactresistance between the electrode 2a, 2b clamping the welded members 1a,1b and to correct the welding pressure in advance to a welding pressureadapted to the current contact resistance between the welded members.Thereafter, the welding current is supplied, and by supplied constantwelding current to the proper current path area obtained as above for arequired period of time, a weld of the desired quality can be obtained.Depending upon the welding conditions, such as the kind of members to bewelded, it is also possible to obtain the desired weld quality byeffecting the detection of the above electrode resistance and thewelding pressure control to ensure the current path area at an initialstage of the welding current supply and then supplying a predeterminedconstant current for a required period of time. However, in this case,it is possible to prevent the formation of surface flash between theelectrodes 2a, 2b and the welded members 1a, 1b or interior flashbetween the welded members 1a, 1b first by supplying a smaller currentthan necessary for welding for one or two cycles at the initial stage ofwelding current supply so as to detect the electrode resistance and tocontrol the welding pressure so that the specified current path area maybe ensured during that time, and next by supplying a welding currentlarge enough to effect welding. This also applies to the case where thewelding pressure is controlled by detecting the electrode resistancewhile the welding current is being supplied. In addition, in the casewhere the electrode resistance is controlled by the welding current, itmay sometimes happen that an abnormally high contact resistance at theinitial stage causes an erroneous operation of the control system. Insuch a case, it is preferable to delay the electrode voltage detectionvia the voltage detection circuit for one or two cycles initially andthen to operate the control system.

Next, the control of the electrode voltage defining the welding currentwill be explained, as will control of the welding pressure between theabove electrodes 2a, 2b. Referring to FIG. 7, the current controlcircuit 16 for the electrode voltage control consists of a thyristor, atriac or other controlling rectifier element. The reference numeral 18denotes a calculating circuit, which inputs and processes the outputsignal from an amplifier 22 that will be described later and the outputsignal from a memory circuit 19, converts the calculated value to afiring phase angle control signal for the current control circuit 16 andoutputs it to the current control circuit 16. The memory circuit 19stores the preceding firing phase angle for one or two cycles of theinitial current supply stage to start the current supply and after oneor two cycles at the onset of current supply actuates the waveformreference point holding circuit 20 to start the control after one or twocycles of current supply. The actual current flow is detected by thecurrent sensing circuit 8a, and the firing phase angle of the preceedinghalf-cycle is stored at respective points in time. The reference numeral20 denotes a waveform reference point holding circuit similar to thewaveform reference point holding circuit 7, which holds for a half-cycleor a predetermined period the peak value of each half-wave electrodevoltage detected by the voltage detecting circuit 6. The referencenumeral 21 denotes a reference voltage generating circuit, in which ispreviously set and stored a reference voltage curve corresponding to thetime variation of the electrode voltage which results in a high-qualityweld, thereby outputting continuously a reference voltage based thereonin synchronism with the timing signal Tp from the current supply timecontrol circuit 17 after the onset of current supply. A differentialamplifier 22 inputs and compares the output voltages of the waveformreference point holding circuit 20 and the reference voltage generatingcircuit 21 and outputs a signal corresponding to the differentialvoltage.

According to the above function, for the initial stage of weldingcurrent supply, irrespective of the value of the output signal from thedifferential amplifier 22, the calculating circuit 18 outputs a firingphase angle control signal based on an initial stage current firingphase angle to control the current control circuit 16 which then outputsa specified current. Accordingly, possible erroneous operation of thecontrol system due to variation of initial stage contact resistance andinitial flash can be prevented. Thereafter, both the output signal fromthe differential amplifier 22 and the firing phase angle data from thepreceeding half-cycle stored in the memory circuit 19 are inputted tothe calculating circuit 18 and the firing phase angle signal is changedso that the welding current may be controlled in the direction in whichthe output voltage of the differential amplifier 22 approaches zero.Thus, the current control circuit 16 controls the welding current, andthe electrode voltage increases when the welding current increases, anddecreases when the welding current decreases. Even if the temperaturedistribution of the two welded members is the same, the electrodevoltage changes when the firing phase of the welding current changes.Therefore, it is necessary to correct the reference voltage curve storedin the reference voltage generating circuit 21 according to themagnitude of firing phase angle. For this reason, the firing phase angleof the actual welding current stored in the memory circuit 19 istransferred to the reference voltage generating circuit 21 through thecalculating circuit 18 to correct the reference voltage curve based onthe phase angle. Keeping pace with the control of electrode voltagebased on the control of welding current, the above electrode resistanceis detected and thereby the welding pressure between the electrodes iscontrolled. As described above, since the welding current and theelectrode pressure are controlled during the welding current supply sothat the electrode voltage varies in accordance with the referencevoltage curve and the electrode resistance varies in accordance with thereference resistance curve, desired weld quality of the weld can beachieved.

According to this configuration, it is possible to widen thecontrollable range of conditions and to exercise adequate control at alltimes in accordance with the condition of the welded members, in caseswhere the conditions of resistance welding, especially the contactconditions among the electrodes 2a, 2b and the welded members 1a, 1b areliable to change, for instance in the case where the welding is appliedto pressed parts or high-tension steel plates. Furthermore, in the casewhere the free ends the of electrodes tips are heavily pressure-worn, itis possible to prevent excessively large nuggets from being formed andexcessive energy (electric power) from being consumed, since the currentpath area between the electrodes and the welded members and between thewelded members can automatically regulated. Although it has been statedhereinabove that the electrode resistance is detected continuously andcompared continuously with the reference resistance curve and theelectrode pressure is controlled in accordance with the differencebetween the resistance values, it is also possible to control theelectrode pressure in accordance with the differences of the measuredelectrode resistance from the reference resistance by detecting theelectrode resistances at the initial stage alone or at specific pointsin time, for instance at the start, the middle and the end of thecurrent supply. In general, when a hydraulic servo system is used forthe control of electrode pressure, a response of about 50 Hz can beeasily obtained; however, when the electrode pressure is controlled atspecific points in time, for instance at the start, at the middle, andat the final stage of the current supply, a penumatic electrode pressuresystem which is slower in response can be used.

Next, the process whereby the welding current supply time isautomatically and optimally controlled will be explained. For thispurpose, an integrator/adder circuit 23, an integral value comparatorcircuit 24 and a reference integral value generator circuit 25 areprovided. The integrator/adder circuit 23 integrates and adds maximumvalues Vc of the electrode voltage at every half-wave outputted from thewaveform reference point holding circuit 20. The integral valuecomparator circuit 24 compares the integral value outputted by theintegrator/adder circuit 23, i.e., the integral value of the electrodevoltage Vc, with the integral value outputted by the reference integralvalue generator circuit 25 previously set such that the desired weldquality will thereby be achieved, and, when they agree, sends a signalthat cuts off the welding current to the current supply time controlcircuit 17 so as to cut off the welding current. By this arrangement,the welding current supply time can be automatically controlled alwaysto the optimum time, and thus a more appropriate weld quality can beachieved.

FIG. 8 illustrates a concrete embodiment of the welding pressure controlcircuit 14. In this figure, the reference numeral 91 denotes a hydraulicpump which is driven by a motor 92, the reference numeral 93, a two-wayvalve, the reference numeral 94, a proportional pressure reduction valvefor adjusting the welding pressure, the reference numerals 95,96, checkvalves, and the reference numeral 97, a relief valve. A hydrauliccircuit is formed by connecting these members to the upper and lowerends of the cylinder chamber of the piston 3 via tubing. The referencenumeral 98 denotes a pressure detector which detects the pressureapplied to the piston 3 on the basis of a differential pressure betweenthe pressure gate ports P₁ and P₂, and the reference numeral 99, a servoamplifier which receives the signal Vd from the above amplifier 12 andoutputs a signal which controls the proportional pressure reductionvalve 94, using a signal Vp representative of the pressure detected bythe pressure detector 98 as a feedback signal. When the two-way valve 93is as illustrated in the figure, the hydraulic pressure is supplied tothe lower end of the cylinder chamber of the piston 3 so as to elevatethe electrode 2a, while, when the two-way valve 93 is switched to thereverse direction in the Fig. by a signal from a control panel (notshown), the hydraulic pressure supplied by the hydraulic pump 91 issupplied through the proportional pressure reduction valve 94 to theupper end of the cylinder room of the piston 3 to push the electrode 2adown as shown by phantom lines, so that the electrode 2a presses thewelded members 1a, 1b in cooperation with the electrode 2b. The weldingpressure is continuously increased and decreased depending upon thesignal Vd, with the proportional pressure reduction valve 94 controlledby the servo amplifier 99 in response to the signal Vd.

When the current path area between the electrodes and the welded membersand between the welded members excessively increases due topressure-wear on the electrode tips, the welding current required tomatch the electrode voltage to the reference voltage value increases,but the welder cannot supply current beyond the current capacitythereof. Therefore, it is preferable to detect the difference betweenthe maximum current suppliable by the welder and the required weldingcurrent and to output a signal when the difference becomes zero, inorder to stop the control function and to display or warn of it. In spotwelding, the shape of the free end of the electrode tips affects therelation between the welding pressure and the current path area,especially the proportionality constant thereof, and, although tips ofany shape may be used, in general it is preferable to use round ordome-shaped tips from the view point of controlling the current patharea by way of the welding pressure. In this controlling system, in thecase where the voltage between the electrode tips deviates from thereference voltage, the differential voltage is detected and the weldingcurrent is changed so that the differential voltage is eliminated, andthis change of the welding current is usually effected by changing thefiring angle of the thyristor within the welding current control unit.The relationship between the differential voltage and the amount ofchange of firing angle of the thyristor is previously set separately inconsideration of the response of the control system, and in general whenthe power source voltage is kept constant, it is unnecessary to changethis relationship. However, in cases where the power source voltagevaries widely with time, for instance when the system is used atmassproduction rate, at the times when the power source voltage widelyvaries it is sometimes difficult to provide such a sufficient change ofthe welding current as to eliminate the differential voltage if thefiring angle of the thyristor for changing the welding current so as toeliminate the differential voltage is set at the same level as beforethe change. Therefore, when the power source voltage changes, it ispossible to ensure good welding quality effectively with the presentcontrol system by changing the relationship between the differentialvoltage and the thyristor firing angle according to the amount of changeof power source voltage, even if the power source voltage changeswidely. Meanwhile, the detection of welding current is carried out inevery case at the primary side of the welding transformer; however, itcan be also carried out at the secondary side. And, it is preferable todetect the welding current on the secondary side in case of multispotwelding, serial spot welding or the like.

FIG. 9 is a block diagram illustrating the configuration of a secondembodiment of the present invention. This embodiment is capable offurther improving the accuracy of control of the welding current supplytime by utilizing the differential voltage integral value integratedover the time when the above described electrode voltage Vc exceeds apredetermined base voltage Vo. In this figure, the same functions asthose in FIG. 7 are shown by the same reference numerals, and thus thedescription of these portions will be omitted. Meanwhile, the departuresfrom the embodiment of FIG. 7 are that a base voltage setting circuit 26is provided in the integrator/adder circuit 23 and that the referenceintegral value generator circuit 25 holds a previously determinedreference voltage integral curve, which represents a high-quality weld,and outputs an instantaneous reference voltage (integral value).

In this second embodiment, the integrator/adder circuit 23 integratesand adds the differential voltage (Vc-Vo) only when the peak value Vc ofeach half-wave of the electrode voltage outputted by the waveformreference point holding circuit 20 exceeds the base voltage Vo which ispreset by the base voltage setting circuit 26. The integral valuecomparator circuit 24 compares the integral value outputted by theintegrator/adder circuit 23, or the integral value from the differentialvoltage (Vc-Vo) when the electrode voltage Vc exceeds the base voltageVo, with an integral value outputted by the reference integral valuegenerating circuit 25 and when the values coincide outputs a signal tothe current feeding time control circuit 17 which cuts off the weldingcurrent. Thereby, the welding current supply time can be automaticallycontrolled to the optimum time under all conditions and thus a moreappropriate weld quality can be achieved.

FIG. 10 is a block diagram illustrating the configuration of a thirdembodiment of the present invention. In accordance with this embodiment,as in the previous first and second embodiments, not only is control ofthe welding pressure, welding current and current supply time effectedusing the differential electrical properties obtained by comparing, ateach sampling interval of one-half cycle, the electrical properties,namely the electrode voltage, electrode resistance and current valuesduring welding under preset optimum welding conditions with the quantityof electricity detected during subsequent welding, but also effected awide range of arithmetic operations as described above concerning theindividual welder itself, welding conditions or variation thereof can beeffected during each half cycle. Accordingly, it is possible to let theelectrical property in question promptly and surely conform to thereference electrical property during welding.

Referring to FIG. 10, the outputs of the waveform reference pointholding circuit 7, 9 and 20 are respectively converted from analog todigital (hereinafter referred to as "A/D") by analog-to-digitalconverters 27, 28 and 29. The output voltage value of the A/D converter27 and the output current value of the A/D converter 28 are inputted toa resistance value calculating circuit 10' in which electrode resistanceis derived by dividing the voltage value by the current value. Thederived electrode resistance is stored via a switch SW₁ in a referenceresistance value memory circuit 11'. With respect to the outputelectrode voltage of the A/D converter 29 on the other hand, only whenit exceeds the base voltage Vo preset by the base voltage settingcircuit 26', the differential voltage (Vc-Vo) is integrated by theintegrator/adder circuit 23' and stored via a switch SW₂ in the integralvoltage value memory circuit 25'. In addition, the output of the A/Dconverter 29 is received via a switch SW₃ by the reference voltagememory circuit 21' to store the electrode voltage. These memory circuits11' , 21' and 25' respectively store the corresponding electricalproperty values for the case of welding under optimum weldingconditions, and may consist of, for example, a magnetic memory or a RAM(Random Access Memory) composed of a semiconductor. In addition, thememory circuit 21' may be connected to a display unit (not shown) forexample, an ordinary pin-type oscilloscope or an electromagneticoscilloscope so that digital values continuously stored may be displayedand observed in the form of voltage waveforms.

In the resistance value comparator circuit 12', a digitalized resistancevalue stored in the reference resistance value memory circuit 11' andthe digital value of the resistance newly detected in the on-goingwelding process are compared at each half-cycle and the differencebetween the two resistances is outputted to the amplifier 13. In theintegral voltage value comparator circuit 24', a digitalized referenceintegral voltage value stored in the reference integral voltage valuememory circuit 25' and a integral voltage value newly obtained in theon-going welding process are compared at each half-cycle and thedifference between the two integral voltage values is outputted to thecurrent supply time control circuit 17. In addition, in the voltagecomparator circuit 22', too, a digitalized reference voltage waveformstored in the reference voltage memory circuit 21' and the digital valueof the electrode voltage newly detected in the on-going welding processare compared at each half-cycle, and the difference between the twovoltages is outputted to a phase conversion data outputting circuit 30.The phase conversion data outputting circuit 30 contains a data bank ofphase conversion values previously chosen so that the digital values ofthe differential voltage and the phase conversion quantity of thecurrent control circuit 16, which controls the welding current,correspond, and when the digital value of the differential voltage isoutputted by the comparator circuit 22', the data bank detects the phaseconversion quantity corresponding to the differential voltage and inputsit to the current control circuit 16. The phase conversion dataoutputting circuit 30 also receives voltage data from the power source15 through a power source voltage monitor unit 31, which will bedescribed later.

The phase conversion data bank of the phase conversion data outputtingcircuit 30 may consist of, for example, a magnetic memory or a ROM (ReadOnly Memory) composed of a semiconductor. In the ROM is stored a phaseconversion data table as schematically shown in FIG. 11. In this figure,the lefthand numerals 1, 2, . . . , m are arranged in terms of theabsolute value of the differential voltage Dv between the referenceelectrical property, for example the electrode voltage Vs obtaining whenthe welded members 1a, 1b are welded previously under optimum weldingconditions, and the electrode voltage Vc detected during the currentwelding. These numerals are determined by first estimating the maximumvalue of variation (range) of the differential voltage Dv by experiment,and then limiting the storage capacity of the phase conversion datatable on the basis thereof, and dividing same into m equal parts. Thenumerals 1, 2, . . . n along the top row (referred to as "table numbersor index addresses") are determined empirically by finding a value withan adequate range with regard to the welding machine, welding materialsand welding condition anticipated to be used and with respect to controlrecovery rate (loop gain), and then dividing the value into n parts.Auuming that the welding current to be used in conjunction withdifferential voltage Dv is Di, the relation between the two can beexpressed as follows:

    Di.sub.p =kDv(p-1)

Here, the symbol k denotes the control recovery rate, and p is thenumber of repetitions of half-cycle periods. That is, the data tablecontains coded values of the firing angle of a silicon control rectifierwithin the current control circuit 16 corresponding to the values of therespective loop gains in the above formula on the basis of thedifferential voltage Dv between the reference electrode voltage Vc andthe electrode voltage detected during subsequent welding. The value isretrieved from the phase conversion data output circuit 30 and outputtedto the current control circuit 16.

The current control circuit 16 is composed of a phase control circuit16a, a thyristor trigger pulse generating circuit 16b, a switchingelement 16c and a silicon control rectifier (SCR) 16d. The output of thephase control circuit 16b, the details of which will be described later,is sent to the thyristor trigger pulse generating circuit 16b. Thethyristor trigger pulse generating circuit 16b generates trigger pulsesduring the operation of the current supply time control circuit 17. Theoutput of the thristor trigger pulse generating circuit 16b is appliedvia the switching element 16c to the SCR 16d. The supplied voltage fromthe ac power source 15 is regulated by the SCR 16d and supplied via thetransformer 4 to the welding electrodes 2a, 2b.

If, by way of example, the absolute value of the output differentialvoltage Dv of the comparator circuit 22' corresponds to "3" in thecolumn of differential voltage indices of the phase conversion datatable, and the separately selected table number (to be described later)is "2", then the firing angle of the SCR 16d would be corrected by anamount corresponding to the value "1" found in the phase conversion datatable at the intersection defined by the two former values. Since thedifferential voltage Dv shown in the phase conversion data table is thethe absolute value thereof, when Dv>0, the output value is positive,when Dv<0, the output value is negative and when Dv=0, the output valueis 0.

Selection of the table numbers 1, 2, . . . , n is made in order tocompensate the quantity of correction of the firing angle of the aboveSCR 16d in cases where the power source voltage varies or the abovevalue of differential voltage Dv deviates beyond a previously supposedallowable range. Considering here cases other than that in which thevalue of the differential voltage Dv deviates beyond the allowablerange, the compensation for the variation of the power source voltage Vcis firstly effected by selecting the table number before the start ofindividual welding processes, and then selecting the compensation basedon the quantity of differential voltage Dv during the welding processes.These controls will be described in detail with reference to FIG. 12.

In the figure, the power source voltage monitor unit 31 constantlymonitors the voltage Vc of the power source 15, and may consist of, forexample, a known digital volt member or the like. The voltage datadigitalized by the power source voltage monitor 31 is supplied to apower source voltage decoder 30a of the phase conversion data outputcircuit 30. The decoder 30a outputs the magnitude of deviation of theinput voltage with respect to the reference voltage value in the form ofclassified plural-level signals. For instance, when the referencevoltage, corresponding to welding under optimum welding conditions, isset at 100 V and the condition required for a shift from a firstlyspecified table number to another is a 5% change in the value of thepower source voltage, the decoder 30a outputs a selection signal thatshifts the table number up by one rank when the power source voltage isin the range of 105-109 V, outputs a selection signal that shifts up bytwo ranks when in the range of 110-114 V, and outputs a selection signalthat shifts up by three ranks when in the range of 115-119 V. Similarly,when the power source voltage is lower than the reference voltage and isin the range of 95-91 V, it outputs a selection signal that shifts thetable number down by one rank, outputs a selection signal that shiftsdown by two and three ranks when in the range of 90-86 V and when in therange of 85-81 V, respectively. In this manner, the selection signalobtained from the decoder 30a is inputted to a data bank 30c and thetable number is selected (specified) in accordance with the shifting dueto the selection signals. Assuming that the table number "2" is selectedat the start of the welding in consideration of the deviation at thattime and the power source voltage is then boosted by 5%, the tablenumber 3, shifted one rank up, is selected while, when the power sourcevoltage is lowered by 5%, when the table number 1 is selected.

In the case where welding is being adequately effected with the tablenumber thus selected, the selection of the differential voltage numberduring subsequent selding is effected (by the differential voltage datacoder 30b) and if the value of the differential voltage Dv between thereference electrode voltage Vs and the electrode voltage Vc detectedduring subsequent welding deviates widely beyond the preset allowablerange due to some variations of conditions, then the table number of thephase conversion data bank 30c which has been already selected on thebasis of the power source voltage is further corrected in accordancewith the differential voltage Dv. In order to cope with this case, adifferential voltage decoder 30d (not shown in the figure) is providedin the phase conversion data outputting circuit 30 which is separatelyactivated by a threshold value, and it is so arranged that, upon receiptof the output of the comparator circuit 22, the table number of thephase conversion data bank 30c may be shifted by the decoder outputobtained. For example, when the differential voltage Dv exceeds 0.3 V,the table number of the phase conversion data bank 30c is automaticallyshifted by one rank by the differential voltage decoder 30d as in thecase where the power source voltage varies. The data table thus devisedis capable of optimally adjusting the firing angle of the SCR 16d.

The phase control circuit 16a within the current control circuit 16 isprovided with a welding current setting circuit, which is set at theinitial stage of the welding by controlling the thyristor trigger pulsegenerating circuit 16b via a welding current setting knob 16e.Meanwhile, the data output received by the phase control circuit 16a andretrieved from the phase conversion data output circuit 30 is sent tothe thyristor trigger pulse generating circuit 16b directly through thephase control circuit 16a unchanged, since it has previously beenconverted to a phase conversion quantity. The trigger pulse of thethyristor trigger pulse generating circuit 16b to which the data outputis sent is digitally shifted by an amount corresponding to the phaseconversion quantity, and the firing angle of the SCR 16d is controlledso as to conform to the standard electrode voltage waveform.

With respect to operational steps of the control function which areeffected by the phase conversion data output circuit 30 constructed asabove, explanations will be made hereinbelow. Firstly, in advance of anactual welding operation, welded members are experimentally welded so asto determine the most appropriate welding conditions (welding current,current supply time and welding pressure) for welding the weldedmembers, and the welding current and current supply time of the weldingconditions are set by the welding current setting knob 16e of the phasecontrol circuit 16a and the welding time (current supply time) settingknob 17a of the current supply time control circuit 17. Thereafter, withthe welded members 1a, 1b clamped and compressed by the upper and lowerelectrodes 2a, 2b which are mounted on the head of the welding machine(not shown), a microswitch provided in the welding machine reacts to apreset welding pressure to send an instruction for starting the currentsupply to the current supply time control circuit 17. In the controlcircuit 17, the welding current is supplied to the welded members 1a, 1bstarting from the instruction of the microswitch for a period of timepreset by the welding time setting knob 16e via the SCR 16d which iscontrolled by the thyristor trigger pulse generating circuit 16b. Thevoltage waveform which is generated between the welding electrodes 2a,2b at this initial welding stage is a voltage waveform that serves asthe standard for the subsequent welding operations, and is held via thevoltage detecting circuit 6 by the waveform reference point holdingcircuit 20. The output of the waveform reference point holding circuit20 is A/D converted at the A/D converter 29 and stored via the switchSW₃ in the memory circuit 21'. The waveform which is stored in thememory circuit 21' after going through this process is referred to as astandard voltage waveform. When the standard voltage wave form optimizedfor the welded members has been stored by way of thepreviously-described operations in the memory circuit 21', the actualwelding operations are carried out with the switch SW₃ opened.

The electrode voltage picked up during the same kind of weldingsubsequently carried out and processed by the waveform reference pointholding circuit 20 and the A/D converter 29 is received by thecomparator circuit 22' and compared with the standard voltage waveformfrom the memory circuit 21'. This comparison is performed at specifiedsampling intervals (half cycle) and the differential value (differentialvoltage) between the two voltage wave forms is derived and sent to thephase conversion data output circuit 30. In the phase conversion dataoutput circuit 30, the preset table number of the phase conversion datatable is first corrected just before welding in accordance with themagnitude of deviation of the power source voltage from the power sourcevoltage monitor 31 and then the phase conversion data (quantity of thephase conversion) of the SCR 16d within the table cell corresponding tothe above differential voltage and to the number thus compensated isretrieved. The retrieved data is inputted to the latter stage phasecontrol circuit 16a to successively regulate the firing angle of the SCR16d via the thyristor trigger pulse generating circuit 16b and theswitching element 16c, and thus the value of the welding current isincreased/decreased each cycle. As a result thereof, the voltagewaveform of the welding electrodes follows the standard voltage waveformand thus the welding quality is always maintained constant.

According to this embodiment, not only can the standard voltage waveformbe traced promptly and surely, even if deviations occur, such as dividedflow due to the proximity of the weld site to a previous weld site, thesurface condition at the weld site or the welding conditions (weldingcurrent, current feeding time and welding pressure), the standardvoltage waveform can still be traced.

FIG. 13 is a block diagram illustrating the configuration of anembodiment of an electrode exchange time display system of the presentinvention. This display system is to detect and display the electrodetip diameter by detecting the electrode resistance R, on the basis ofthe fact that the electrode resistance R and the reciprocal l/S of thecurrent path diameter (area) are proportional as described above andthat the current path diameter is substantially the same as theelectrode tip diameter. In the figure, the reference numeral 32 denotesa current path diameter calculating circuit, which is connected to theresistance value calculating circuit 10 or 10' of the embodiments shownin FIGS. 7, 9 and 10. And, on the basis of the electrode resistancevalue outputted from the calculating circuit 10 or 10', the current pathdiameter is derived by the current path diameter calculating circuit 32.The reference numeral 33 denotes a reference current path diametergenerating circuit, which outputs a signal at a specified point in time,for example, at one point in time of the initial stage of currentsupply, corresponding to a previously defined electrode tip diameterrepresenting pressure-worn electrode tips 2a, 2b which have reached thetime of exchange. The outputs of the current path diameter calculatingcircuit 32 and the reference current path diameter generating circuit 33are compared by the current path diameter comparator circuit 34. Whenthe current path diameter exceeds the value of the reference currentpath diameter, that fact is sent to be displayed on a display unit 35,such as a lamp, buzzer and the like. Accordingly, the arrival ofexchange time for the electrodes can be indicated to the operator or thelike.

Although the above respective embodiments have been described withregard to the case in which the welding is carried out using alternatingcurrent, and, according, the sampling value control, which controls thewaveform, is carried out by taking the peak value of each half-wave asthe reference point thereof it is also possible to execute repetitivecontrol in the case that the welding is performing by means of directcurrent. Furthermore, though the descriptions have all referred to caseswhere the present invention is applied to the spot welding, the presentinvention can be also applied to other resistance weldings, namely, toprojection welding, seam welding, flash welding, upset welding and othertypes of welding. Furthermore, in the case of multispot welding, serialspot welding and the like in which plural electrodes are employed in asingle welding machine, it is possible to independently and easilycontrol the quality of each weld as required by individually detectingand controlling the electrode pressure at each weld site.

As will be apparent from the above explanations, in accordance with thecontrol system of the present invention, since the welding pressurebetween the electrodes clamping the welded members or the weldingpressure and the welding current are controlled by detecting theresistance between the electrodes or both the resistance and the voltageduring the welding process to that the desired quality of the resistanceweld is assured subsequent processing of substandard welds and completeloss of finished products can be nearly eliminated, and thus the rejectrate of the products can be greatly reduced and the operating efficiencycan be improved. In addition, the control of welding pressure betweenthe electrodes is particularly effective for preventing flashes, whichimproves safety and prevents degradation of appearance and quality.

Furthermore, in the case where the power source voltage greatly variesor in the case where the difference between the reference voltagewaveform and electrode voltage detected during subsequent welding islarge, the standard voltage waveform can be also traced, and thus verygood welding quality can be ensured.

Moreover, since the electrical properties corresponding to welding underoptimum welding conditions are measured and automatically stored, onlyvery simple preparation is required before starting the actual weldingeven in low-quantity manufacture of different kinds of products, and,the electrical properties are controlled at every half-cycle duringcurrent supply, the welding quality can be assured not only in the caseof resistance welding of the members requiring a relatively long currentsupply time, but also in the case of electronic parts and the likerequiring a very short current supply time.

We claim:
 1. A method for controlling resistance welding comprising thesteps of:holding the portion of a material to be welded between a pairof electrodes under a pressure; applying an initial current having amagnitude less than that of a welding current for said electrodes inadvance of performing welding; detecting the resistance between saidelectrodes in the presence of said initial current; controlling thepressure on said electrodes on the basis of the detected resistance soas to adjust the resistance between said electrodes to a predeterminedresistance; applying a welding current between said electrodes toperform resistance welding; detecting the voltage between saidelectrodes while the welding current is applied thereto; controlling thewelding current flowing between said electrodes to adjust the voltagedetected between said electrodes to a predetermined voltage; andmeasuring the period during which said welding current is applied andinterrupting said welding current when said measured period reaches apredetermined time period.
 2. A method for controlling resistancewelding comprising the steps of:holding a welding site of a material tobe welded between a pair of electrodes; applying an initial currentsmaller than a welding current to perform resistance welding between theelectrodes in advance of performing welding; detecting a resistancevalue between said electrodes under the presence of the initial current;controlling the pressure of the electrodes on the basis of the detectedresistance value to adjust the resistance value between said electrodesto a predetermined initial resistance value; applying the weldingcurrent between said electrodes; detecting the resistance value betweensaid electrodes while the welding current is applied; controlling thepressure in accordance with the resistance value during application ofthe welding current to maintain the resistance value between saidelectrodes to a given resistance value having a given relationship tosaid initial resistance value; detecting the voltage between saidelectrodes while the welding current is applied; controlling the currentvalue to be applied between said electrodes to adjust the voltagedetected between said electrodes to a predetermined voltage; andmeasuring a time period during which said welding current is applied andinterrupting the welding current when the measured time period reaches apredetermined time period.
 3. A method for controlling resistancewelding comprising the steps of:holding a welding site of a material tobe welded under pressure between a pair of electrodes; applying aninitial current between said electrodes, which initial current has avalue smaller than that of a welding current, before applying saidwelding current; detecting an initial resistance value between saidelectrodes with respect to said initial current before applying saidwelding current; controlling the pressure between said electrodes to acontrolled pressure depending upon the detected initial resistance valueto adjust the initial resistance value to a predetermined resistancevalue; applying said welding current between said electrodes in thepresence of the controlled pressure between said electrodes;continuously detecting the resistance value between said electrodeswhile applying said welding current thereto; continuously controllingsaid pressure on the basis of the detected resistance value to adjustthe resistance value between said electrodes so as to be constantlymaintained within a given allowable range with respect to a givenresistance value having a given relationship to said predeterminedresistance value; continuously detecting the voltage between saidelectrodes while applying said welding current during application of thecontrolled pressure; controlling the current value applied between saidelectrodes on the basis of the detected voltage to adjust the voltage soas to be constantly maintained within a given allowable range withrespect to a predetermined voltage; and measuring the time period duringwhich the welding current is applied and interrupting the weldingcurrent when the measured time period reaches a predetermined timeperiod.
 4. A method for controlling resistance welding comprising thesteps of:pressing a pair of electrodes under pressure onto a weldingsite of a material to be welded; applying an initial current having amagnitude smaller than that of a welding current between saidelectrodes, before applying said welding current; presetting apredetermined resistance value and a predetermined voltage; applyingsaid welding current between said electrodes; detecting the magnitude ofsaid welding current and said initial current at given times; detectingthe voltage between said electrodes at the given time; arithmeticallydetermining an initial resistance value on the basis of a detectedinitial current value and the detected voltage and comparing thedetermined initial resistance value with said predetermined resistancevalue to obtain the difference therebetween; adjusting said pressurebetween said electrodes depending upon the determined difference betweensaid determined and predetermined resistance values so that saiddifference is maintained smaller than a given value; comparing thedetected voltage with said predetermined voltage to determine a voltagedifference; controlling the welding current to be applied on the basisof the determined voltage difference so as to keep the differencesmaller than a given value; and measuring a period of time during whichsaid welding current is applied and interrupting said welding currentwhen the measured period of time reaches a predetermined period of time.5. A controlling apparatus for resistance welding comprising:first meansfor detecting a first resistance between a pair of welding electrodes byapplying an initial current having a value smaller than that of awelding current before supplying the welding current to the weldingelectrodes, between which members to be welded are pinched, comparingthe detected resistance to a predetermined reference resistance tocontrol the pressure between the welding electrodes on the basis of thedifference between the detected and predetermined resistances, detectinga second resistance between said welding electrodes while supplying thewelding current, and comparing the detected second resistance with agiven resistance having a given relationship to said predeterminedresistance to control the pressure between said welding electrodes onthe basis of the difference of the detected second and givenresistances; second means for detecting the voltage between said weldingelectrodes while supplying the welding current, comparing the detectedvoltage with a predetermined reference voltage and controlling thewelding current on the basis of the difference between the detected andreference; and third means for interrupting the welding current when apredetermined current supply time has elapsed to control the period oftime for carrying out resistance welding.
 6. A controlling apparatus forresistance welding which comprises first means for detecting a firstresistance between a pair of welding electrodes by supplying an initialcurrent having an amplitude smaller than that of a welding currentbefore the start of supply of welding current to the electrodes betweenwhich members to be welded are pinched, comparing the detected firstresistance to a predetermined first reference resistance to control thepressure between the welding electrodes in accordance with thedifference between said detected and reference first resistances,detecting a second resistance between said welding electrodes whilesupplying the welding current thereto, and comparing the detected secondresistance to a predetermined second reference resistance having a givenrelationship to said first predetermined reference resistance to controlthe pressure between the above welding electrodes in accordance with thedifference between said detected second resistance and secondpredetermined reference resistance, means for detecting the voltagebetween said welding electrodes while supplying the welding current,comparing the detected voltage to a predetermined reference voltage andcontrolling the welding current in accordance with the differencebetween said detected and reference voltages, means for interrupting thewelding current when a predetermined current supply time has elapsed tocontrol the supply time, and means for detecting the time integral ofsaid detected welding electrode voltage, and interrupting the weldingcurrent when the detected integral has reached a predetermined referenceintegral voltage to control the supply time.
 7. A controlling system inresistance welding which comprises:first means for detecting theresistance between a pair of welding electrodes by supplying an initialcurrent having an amplitude smaller than that of a welding currentbefore supplying of the welding current to the electrodes between whichmembers to be welded are pinched, comparing the detected resistance to apredetermined reference resistance to control the pressure between thewelding electrodes on the basis of the difference between theresistances, detecting the resistance between the welding electrodeswhile supplying the welding current thereto, and comparing the detectedresistance with a predetermined reference resistance to control thepressure between the welding electrodes on the basis of the differencebetween the detected and reference resistances; second means fordetecting the voltage between the welding electrodes while supplyingsaid welding current, comparing the detected voltage with apredetermined reference voltage and controlling the welding current onthe basis of the difference between said detected and reference voltage;and third means for integrating said difference voltage when thedetected electrode voltage exceeds a predetermined reference voltage andinterrupting the welding current when the integral has reached apredetermined reference integral to control supply time.
 8. Acontrolling system in resistance welding according to any one of claims5, 6 and 7 which further comprises means (32, 33, 34) for detecting acurrent supply path on the basis of the resistance between the abovewelding electrodes and displaying the arrival of replacement timing forthe welding electrodes when the free end diameter of the weldingelectrode indicated by the detected supply path has reached apredetermined free end diameter of the welding electrode.